VOX AC30 / 6 Vibe Channel 'ain't right'

[Buddha's Guitar Tech]

i've been doing up an ancient AC30 with good results, but something perplexes me. Having rarely played one of these oldies, i'm not sure how it 'should' sound.

After getting rid of lots of noise, finding dry joints, replacing coupling caps, plate and cathode resistors, i find the vibe/trem channel is quieter than the others, with a lifeless sort of tone. No real body, almost transistor-ish.

Could this possibly be normal? After all, there is a lot of tone networking going on nearby V7.

Should i be looking closer to V8, the 12AU7..?

i have the Tube Amp Book (4.1) with the AC30.36 schematic on page 648.

[Structo]

You got me, I have never played a real AC30.

But I would love to see some pictures of it, inside and out.

[Reeltarded]

Something is wrong. I know not what that is, but yep, something is wrong.

[Buddha's Guitar Tech]

So you're telling me something is wrong?

The tone network for this channel is not your run-of-the-mill Fender/Marshall thing. The others don't have tone networks, and i suspect that this channel needs a second stage because of the drain introduced by such a network.

i'm finding that the second triode stage of the vibe channel is giving me negative amplification. That is to say, the sine wave on the plate is smaller than the sine wave fed to its grid. Urgh, maybe i have to replace more resistors, this time in the network.
i would expect it to be the same size.

More fiddling for me.

Shall of course post some pictures for your pleasure.

[Masco]

The V/T channel in the older AC-30s is usually the weakest of the 3, but you should have some gain on that second stage. You really have to go through that circuit w/ a fine tooth comb. Those old resistors drift way up out of spec.
Since you have a signal gen and o'scope, disable the oscillator by using the footswitch (or lifting the 1M R from ground in the osc feedback network), and scope out each signal path. There are multiple signal splits from that second triode from the plate and cathode and everything gets mixed back together(including the LFO signals) at the modulator inputs. In the "trem" setting, one of the modulator inputs is shorted to ac ground through a 0.1uF cap. Make sure you have the channel on the "vibe" setting, so the signal gets through to both inputs of the modulator when you're scoping that out.
The V/T channel in an AC-30 makes up most of the circuitry in the amp. Fun stuff!!!
Let me know what you find.

[Buddha's Guitar Tech]

Thanks Masco.

Starting at the PI grid, i can see the sine wave is misshapen, as if the bottom half of the wave has been rounded off. That certainly helps explain the lack of volume, but finding it was a different matter. (i checked with both other channels, they look good.)
i've already replaced all plate & tail resistors for the PI so we should be in good shape there.

i can get a decent sine shape at the PI grid when i disable C26 & C27. i realise they lead to the grids of the mod valve, but they also lead off to a tone network, so perhaps a value has drifted there.

i'm running out of spare parts, i'll have to order some.

Anyway, it's now at a point where it seems the volumes are about the same, but the normal channel has tons and tons of body, whereas the vibe channel really lacks warmth. The brilliant channel is 'meh' i suppose.

And again, i have to wonder if it is 'meant' to sound like this.

[lions den]

i can't speak to the technical side of the AC30, but I have a '62 6 holer that sounds great. I'll check it out tonight and pay more attention to the differences in tone at the VT channel. I'll see about pulling it apart and maybe taking some pics as well

thanks,
sean

[Buddha's Guitar Tech]

You dirty tease.

[Zippy]

Have you already swapped out the tubes to ensure that the tubes are correct and good?

Tensioned the sockets?

Have you checked voltages?

Any toasted looking components?

[Buddha's Guitar Tech]

We're way beyond that.

[Zippy]

We're way beyond that.

OK. Great. Sometimes we see people that are way beyond that and haven't paid attention to it. Jes checkin'...

[renshen1957]

i've been doing up an ancient AC30 with good results, but something perplexes me. Having rarely played one of these oldies, i'm not sure how it 'should' sound.

After getting rid of lots of noise, finding dry joints, replacing coupling caps, plate and cathode resistors, i find the vibe/trem channel is quieter than the others, with a lifeless sort of tone. No real body, almost transistor-ish.

Could this possibly be normal? After all, there is a lot of tone networking going on nearby V7.

Should i be looking closer to V8, the 12AU7..?

i have the Tube Amp Book (4.1) with the AC30.36 schematic on page 648.

Hi, I couldn't locate my copy of A Service Engineer's Guide to the Vox AC30 Valve Amplifier by Steve Grosvenor, however from memory the trem channel has only one amplification stage and a volume knob; the rest of the tube stages are involved in the oscillator circuit.

Any idea as to the amps vintage? A picture of the exterior would help.

I don't know if you have an AC30 with a top boost circuit (with additional tubes). The normal channel is voiced different from the trem channel.

Do you have a normal, treble (about 1/3 of all vintage voxes made), or bass version?


I am at work, so no sound on the computer, vintage amps, the first link is a survey of the amp

http://www.youtube.com/watch?v=XVPfGWJf ... re=related

http://www.youtube.com/watch?v=5oGwHJlD ... re=related

Comparison between vintage and a reissue:

http://www.youtube.com/watch?v=QNktNC37 ... re=related


Best REgards

STeve

[lions den]

@Buddha,
I didn't notice a huge tone difference when going to the VT Channel on my AC30. The vibrato setting is definitely thinner sounding than the tremolo channel.

Here is a quick simple video clip. Set the brilliant, normal, and VT Channels to the same volume level, and played quick chords as I moved the guitar down the line in that order. Once on the VT channel it starts with vibrato setting, then over to tremolo, then back to vibrato, and finally back to the brilliant channel. This is a non-TB version.

edit: put some pics up as well, it obviously needs a dust cleaning Video is at the end of the pics. AC30/6 Ser. #02962

i have not pulled the chasis out before to take pics under the board. If anyone MUST see the guts fully exposed post a message and I'll work to see if I can take her apart further.

this was done quickly on an ipad, so sorry for the overall quality, hope you get the idea in general of tones.


http://gallery.me.com/sean.joplin/100144

[Buddha's Guitar Tech]

Mr. Lion's Den, thanks very much for the info and your time.
i notice on the video clip that when you play the VT channel, there's no mic distortion - this tells me that it is in fact quieter than the other two, provided the volumes are set the same. Or maybe the normal channel is fatter? (That's what's going on here, anyway.)

cheers

[Reeltarded]

Don't look at the volume controls. Turn each up until they sound equal.

The vib channel will always be a little higher to sound equal, but not that much. I should have to go to the studio this afternoon for a bit, I will take a picture of what the '63 looks like with the channels at similar volume. It's something like 12 o'clock normal, near 12:30 and a teeny on the vib, just off the top of my head.

It's not thin sounding though... approaching snorting beast on either at that volume.